The long-term goal of this project is to understand how the dysregulation of certain keratinocyte functions, by altering the cells' interactions with te skin microbiome, can impair epidermal barrier function, leading to features resembling those of atopic dermatitis (AD) and the development of high serum levels of IgE. Mouse RabGEF1 was discovered as a negative regulator of mast cell activation, and mice globally deficient in RabGEF1 rapidly develop AD-like skin [pathology]. However, conditional deletion of Rabgef1 specifically in mouse keratinocytes is sufficient to drive AD-like skin [pathology] and the development of high levels of serum IgE (as is also seen in AD patients). Moreover, the severe skin disease observed in RabGEF1- deficient mice critically depends on keratinocyte-intrinsic expression of MyD88, an adaptor molecule that mediates signaling by several Toll-like receptors (TLRs). [Expression of RabGEF1 protein] is markedly decreased in lesional skin from patients with AD as compared to healthy skin, whereas MYD88 mRNA expression is significantly increased in lesional skin specimens from AD patients. Since skin is continuously exposed to microorganisms that are a rich source of TLR ligands, we will evaluate to what extent the skin microbiome and keratinocyte TLRs can contribute to the MyD88-dependent AD-like skin [pathology] observed when keratinocytes lack RabGEF1. We will use RabGEF1-deficient mouse and human keratinocytes to analyze in vitro the molecular mechanisms by which RabGEF1 regulates MyD88-dependent signaling pathways. Finally, we will assess how keratinocyte-restricted [complete or partial] reductions in RabGEF1 expression can influence systemic sensitization to allergens via the skin, with the later development of allergic inflammation in distant organs such as the lung. We propose three specific aims to test the General Hypothesis: The development of skin pathology with features of atopic dermatitis, and elevated serum IgE, in RabGEF1-deficient mice reflects the ability of RabGEF1 to maintain skin [barrier and immune] functions by down-regulating pathways initiated by MyD88-dependent signaling in keratinocytes. Aim 1: Define the contributions of MyD88, Toll-like receptors (TLRs), and the skin microflora in the AD-like skin [pathology] induced by keratinocyte-specific Rabgef1 deletion in vivo. Aim 2: Determine the mechanisms by which RabGEF1 negatively regulates MyD88-dependent functional responses and signaling in keratinocytes. Aim 3: Determine the mechanisms by which RabGEF1 deficiency in keratinocytes influences skin sensitization to allergens and the development of the atopic march. This project will clarify how an intrinsic impairment in keratinocyte function due to diminished RabGEF1 can contribute to AD-like skin [pathology]. Ultimately, pursuing such work may enable the discovery of novel therapeutic targets for the prevention or treatment of AD and perhaps other atopic or inflammatory disorders.